The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to a method for shifting motion-based artifacts from the primary region of interest in the central field of view within an image field of view (FOV) to the side parts of the FOV by manipulating the order of a segmented k-space acquisition. It will be described with particular reference thereto. It will be appreciated, however, that the invention is also amenable to other like applications.
Magnetic resonance imaging is a diagnostic imaging modality that does not rely on ionizing radiation. Instead, it uses strong (ideally) static magnetic fields, radio-frequency (RF) pulses of energy and magnetic field gradient waveforms. More specifically, MR imaging is a non-invasive procedure that uses nuclear magnetization and radio waves for producing internal pictures of a subject. Three-dimensional diagnostic image data is acquired for respective xe2x80x9cslicesxe2x80x9d of an area of the subject under investigation. These slices of data typically provide structural detail having a resolution of one (1) millimeter or better.
The data for each slice is acquired during respective excitations of the MR device. Ideally, there is little or no variation in the phase of the nuclear magnetization during the respective excitations. However, movement of the subject (caused, for example, by breathing, cardiac pulsation, blood pulsation, and/or voluntary movement) and/or fluctuations of the main magnetic field strength may change the nuclear magnetization phase from one excitation to the next. This change in the phase of the nuclear magnetization may degrade the quality of the MR data used to produce the images.
When utilizing MRI to produce images, a technique is employed to obtain MRI signals from specific locations in the subject. Typically, the region that is to be imaged (region of interest) is scanned by a sequence of MRI measurement cycles, which vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques. To perform such a scan, it is, of course, necessary to discriminate NMR signals from specific locations in the subject. This is accomplished by employing gradient magnetic fields denoted Gx, Gy, and Gz. These gradient magnetic fields are static magnetic fields along the x, y, and z axes exhibiting a gradient along the respective x, y and z axis. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be altered and the location is encoded in the resulting NMR signals.
Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Most NMR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences, which have a very short repetition time (TR) and result in complete scans, which can be conducted in seconds rather than minutes. For example, when applied to cardiac imaging, a complete scan of a series of images showing the heart at different phases of its cycle or at different slice locations can be acquired in a single breath-hold.
There are two common techniques for acquiring cardiac MR images. The first is a prospectively gated, single-phase, multi-slice conventional spin echo sequence. In each cardiac cycle, data at different spatial locations are acquired with the same k-space phase encoding value. Images at the different spatial locations are then acquired at different temporal phase of the cardiac cycle.
In gated spin echo, data for each slice location is acquired at a fixed delay from the cardiac R-wave. With variations in the cardiac rhythm, the heart may be at a different phase of the cardiac cycle when data is acquired even though the cardiac delay time is the same. Normal variations of the cardiac cycle usually result in disproportionately larger changes in the diastolic portion of the cardiac cycle, and gated spin echo images acquired at the end of the cardiac cycle often exhibit blurring or ghosting artifacts.
Another disadvantage of gated spin echo is that images at different slice locations are acquired at different cardiac phases. Hence, it may be difficult to relate information from one spatial location to the next as the heart is pictured at different phases of the cardiac cycle. Furthermore, small structures may also be missed due to inadequate temporal and spatial coverage. Motion of the heart during the cardiac cycle can also lead to image contrast variations from slice to slice due to differential saturation or inter-slice cross talk.
A short TR gated gradient echo pulse sequence may be used to acquire (cine) images at multiple time frames of the cardiac cycle. As described in U.S. Pat. No. 4,710,717, conventional cine pulse sequences run asynchronously to the cardiac cycle with the phase encoding value stepped to a new value at each R-wave trigger. In CINE, each RF excitation pulse is applied at the same spatial location and repeated at intervals of TR in the cardiac cycle. Since the sequence runs asynchronously, the RF excitation pulses may occur at varying time delays from the R-wave from one cardiac cycle to the next. On detection of the next cardiac R-wave, the acquired data from the previous Rxe2x80x94R interval are resorted and interpolated into evenly distributed time frames within the cardiac cycle. This method of gating is also known as retrospective gating as the data for the previous Rxe2x80x94R interval is resorted only after the current R-wave trigger is detected.
The cardiac cycle is divided into equal time points or frames at which images of the heart are to be reconstructed. In order to reconstruct images at each of these time points, data acquired asynchronously is linearly interpolated to the pre-determined time points in the cardiac cycle. In order to account for variations in the cardiac Rxe2x80x94R interval during the scan (from changing heart rate), the interpolation varies from cardiac cycle to cardiac cycle, depending on the Rxe2x80x94R interval time. This method allows reconstruction of images at any phase of the cardiac cycle, independent of variations in heart rate. As in gated spin echo, only one k-space phase encoding view is acquired per heartbeat. The total image acquisition time is then on the order of 128 heart beats.
Faster scan times can be achieved by segmenting k-space and acquiring multiple phase encoding k-space views per Rxe2x80x94R interval. The scan time is accelerated by a factor equal to that of the number of k-space views acquired per image per Rxe2x80x94R interval. In this manner, a typical cine (e.g. movie or temporal series of images) acquisition with a matrix size of 128 pixels in the phase encoding direction can be completed in as little as 16 heart beats, when 8 k-space views per segment are acquired.
Multiple phases of the cardiac cycle can be visualized by repeated acquisition of the same k-space segment within each Rxe2x80x94R interval but assigning the data acquired at different time points in the cardiac cycle to different cardiac phases. Thus, the cardiac cycle is sampled with a temporal resolution equal to the time needed to acquire data for a single segment, such that the temporal resolution equals the views per second, vps multiplied by the TR, where vps is the number of k-space lines per segment, the TR is the pulse sequence repetition time. The total scan time is then given as: the quantity yres divided by vps, times the Rxe2x80x94R interval time, where yres is the number of phase encoding views in the image. Typically, an image utilizes 128 or more phase encoding views, and 8 views per segment is also often used.
In segmented k-space scans, the total scan time can be substantially reduced by increasing the number of views per segment (vps). However, this is at the expense of reducing the image temporal resolution. As described in U.S. Pat. No. 5,377,680, the image temporal resolution can be increased by sharing views between adjacent time segments to generate images averaged over different time points. The true image temporal resolution is unchanged but the effective temporal resolution is now doubled. View sharing can thus increase the number of cardiac phase images reconstructed without affecting the manner in which the k-space data is acquired.
Prospectively gated, segmented k-space sequences have become popular for cardiac imaging mainly because images can be obtained in a breath-hold and therefore do not suffer from respiratory artifacts. Images are formed by acquiring data over a series of heartbeats with data acquisition gated to the QRS complex of the ECG. For images to be acquired properly, using current methods, the duration of image acquisition must be less than or equal to the duration of the shortest expected Rxe2x80x94R interval. In practice, this usually means that the last 10-20% of diastole (.about. 100-200 msec for a heart rate of 60 bpm) is not acquired.
Another problem with many current cardiac-gated sequences is that they sort data based on the time elapsed since the QRS complex. As described in U.S. Pat. No. 4,710,787, this strategy assumes that cardiac phase is directly proportional to time. However, in practice the relationship between cardiac phase and the time elapsed since the QRS is not strictly linear. For example, consider sinus arrhythmia where there is a normal, physiologic change in heart rate that accompanies respiration. The time between the QRS complex and end-diastole is longer for those heart beats with longer Rxe2x80x94R intervals and in this case, end-diastole is better defined relative to the following (rather than the preceding) QRS complex. This can be seen readily in the normal ECG where the P wave (which signifies atrial contraction) is better correlated temporally to the following (rather than the preceding) QRS complex. This variation in the Rxe2x80x94R interval and the fact that a specific cardiac phase occurs at a different delay time from the R-wave with this variation, leads to image blurring in fast segmented k-space pulse sequences and also in conventional cine pulse sequences.
Another method for prospectively gating and retrospectively sorting MR imaging data acquired during successive cardiac cycles was disclosed in U.S. Pat. No. 5,997,883. In this patent, a cardiac gating signal is produced and time-stamped MR data is continuously acquired during successive cardiac cycles to reduce image blurring in fast segmented k-space and cine acquisitions and more efficiently acquire MR data. A cardiac cycle systolic period and a cardiac cycle diastolic period are determined for each cardiac cycle and the time stamp associated with the acquired MR data is correlated with a systolic cardiac phase or a diastolic cardiac phase. Images are reconstructed at specified cardiac phases using MR image data which is acquired during successive cardiac cycles and which is selected on the basis of its correlated cardiac phase.
When acquiring a multiple images over time (e.g., a temporal series or movie), particularly of a patient""s beating heart, motion paired with finite data acquisition duration limits the temporal resolution and leads to specific motion-related artifacts. For some sequences, these artifacts occur mainly in the center-half of the field of view (FOV), overlapping the object of interest, e.g., the heart. Because the center of the FOV is primarily the focus of the imaging, clinical image analysis may be impaired by artifacts such as motion-induced artifacts. These artifacts are generally caused by blood flow or simply cardiac motion with successive beats.
The above discussed and other drawbacks and deficiencies are overcome or alleviated by a system and method for shifting of motion based artifacts in images produced with an magnetic resonance imaging system, comprising: a magnetic resonance imaging system configured to: select a segment of a plurality of segments comprising a selected number of k-space lines of k-space data for a temporal series and select a time interval from a plurality of time intervals for acquisition. The magnetic resonance imaging system is also configured to acquire N sets of k-space lines comprising every Nth k-space line of the segment for successive 1/N portions of the time interval and repeating the acquisition for successive sets of the N sets of k-space lines and wherein N is an integer greater than one. The acquisition is repeated for each time interval of the plurality of time intervals and for each segment of the plurality of segments. The k-space data are reconstructed employing a time-weighted average based upon respective time of acquisition for the k-space lines from the k-space data acquired employing the magnetic resonance imaging system.
Also disclosed is a method of acquiring k-space data produced with an magnetic resonance imaging system, comprising: selecting a segment of a plurality of segments comprising a selected number of k-space lines of k-space data for a temporal series and selecting a time interval from a plurality of time intervals for acquisition. The method also includes: acquiring N sets of k-space lines comprising every Nth k-space line of the segment for successive 1/N portions of the time interval and repeating the acquisition for successive sets of the N sets of k-space lines and wherein N is an integer greater than one. The method further includes repeating the acquiring for each time interval of the plurality of time intervals and for each segment of the plurality of segments. The k-space data are reconstructed employing a time-weighted average based upon respective time of the acquiring of k-space lines from the k-space data acquired employing the magnetic resonance imaging system.
Also disclosed is a storage medium encoded with a machine-readable computer program code including: instructions for causing a computer to implement the abovementioned method for shifting of motion based artifacts in images produced with an magnetic resonance imaging system.
Further disclosed is a computer data signal comprising code configured to cause a processor to implement the above mentioned method for shifting of motion based artifacts in images produced with an magnetic resonance imaging system.
The above discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings.